According to both the IMF and the World Bank, the world's 10 largest economies in 2006 were, in order, the EU, the US, Japan, Germany, the PRC, the UK, France, Italy, Canada, Spain and Brazil. Very mixed economies, every one. I really don't see how this supports the idea that they're being "dragged down."
Also, how does the fact that the US economy does well in comparison to other more socialist countries in any way show that removing the socialist controls we have now would not improve our economy even more?
I maintain that it doesn't support your argument that our economy is being dragged down. You may argue that it's not growing at the rate you think it would, and I can respond by arguing that the resulting society isn't one I'd want to live in -- that's a different discussion. But the general trend of growth year over in real GDP (http://www.ers.usda.gov/Data/Macroeconomics/Data/HistoricalR...) is not evidence of being "dragged down." The trend is broadly up.
Regarding your second point, this is a canard. The fact that the 10 largest, most successful economies in the world (in fact, most if not all of the top 20) incorporate elements of socialism (most to a much greater degree than the US) demonstrates that mixed economies can, and do, succeed. Your initial argument at least implies that a mixed economy is not a successful one. If I read that incorrectly, please tell me, because by most metrics, mixed economies are not only surviving, but prospering.